Wastewater Treatment

Municipal WW Management Systems

Sources of Wastewater

Processing at the Source

WastewaterCollection

Transmissionand Pumping

Treatment Reuse/Disposal

Sources & Types of WW

Domestic Wastewater– from residences, commercial & institutional– flows ~70-90% of water supplied

Industrial Wastewater– highly dependent on industry

Infliltration/Inflow– enters through leaks, foundation drains, etc.

Stormwater– for combined sewers - largely in older cities

Typical WW Characteristics

Parameter Conc.BOD 250 mg/LTSS 250 mg/LCOD 500 mg/LAmmonia 30 mg/LTOC 100 mg/LChloride + 50 mg/L

1972: Federal Water Pollution Control Act

PL 92-500 subsequently amended and now called the Clean Water Act– established water quality goals “fishable &

swimmable” and timetable– established National Pollution Discharge

Elimination System (NPDES)– construction grants for WW treatment

required secondary treatment (30/30)– 30 mg/L BOD30 mg/L BOD55

– 30 mg/L TSS30 mg/L TSS

TREATMENT CLASSIFICATION

PRIMARY– Removal of solids

SECONDARY– Removal of organics

TERTIARY– Removal of nutrients (N and P)

Conventional WW TreatmentBiological ProcessPreliminary

Treatment

SecondarySedimentation

SludgeSludge

Disinfection

PrimarySedimentation

SludgeSludge

Preliminary Treatment

To remove materials that will interfere with subsequent treatment– Coarse Screening (bar racks)– Medium Screening– Comminution– Flow measuring– Pumping– Grit removal– Pre-aeration

Bar Racks

Metal bars spaced a few cm apart across water flow

mechanical or manually cleaned size of unit set by approach velocity

– 0.6-1.0 m/s for mechanically cleaned– 0.3-0.7 m/s for manually cleaned

see Fig 10.4 and example 10.2 on pg. 311

Grit Removal

Grit chambers intended to remove sand, cinders, gravel that may enter system by cracks in pipes, inflow etc.

Grit can cause excess wear in pipes and pumps

small sedimentation tanks; designed with the help of Stoke’s Law

no organics removal

Grit Chamber

Primary Sedimentation

Purpose: to remove suspended solids (smaller than grit, and less harmful)

Typical efficiency– 67% TSS removal– 33% BOD removal

Design parameters– overflow rate– weir loading rate– detention time

Primary effluent is Primary effluent is largely composed of largely composed of soluble and colloidal soluble and colloidal organics which can be organics which can be converted to settleable converted to settleable microbial solids and microbial solids and COCO22 by biological by biological

treatmenttreatment

Primary Sedimentation– Primary Treatment– Removes ~50% of suspended solids

Parameter Design Range TypicalValue

OverflowRate

35-45 m/d800-1200 gal/ft2/d

40 m/d1000 gal/ft2/d

DetentionTime

1.5-2.5 h 2 h

Weir loadingrate

125-500 m2/d10,000-40,000 gal/ft/d

275 m2/d20,000 gal/ft/d

Primary Sed. Tank

Primary Clarifier: Center Feed

Secondary Treatment

Generally includes some biological process plus secondary clarification

Required under PL92-500 Converts soluble and colloidal organic

materials to biomass and CO2

Biological Treatment

Suspended Growth– Activated Sludge

» Conventional, Extended Aeration, Contact Stabilization

– Aerated lagoons– Aerobic digestion

Attached Growth– Trickling Filters– Rotating Biological Contactors

Suspended Growth Systems

SludgeSludge

SecondarySedimentation

Return Activated SludgeReturn Activated Sludge

WasteWasteActivated SludgeActivated Sludge

Aeration TankAirAir

Attached Growth: Trickling Filters

Rocks or Plastic Media

Underdrain

Rotary Distributor

With rocks, depth is limited to 2-3 m because of oxygen needsWith rocks, depth is limited to 2-3 m because of oxygen needs

Air

Rotating Biological Contactors

• Drum diameters are typically 10-12 ft.Drum diameters are typically 10-12 ft.• Rotation speed: ~1.5 rpmRotation speed: ~1.5 rpm• May be in several stagesMay be in several stages• No flow recycleNo flow recycle• Requires pilotingRequires piloting

Secondary Clarifier

Sludge Disposal

Thickening– gravity, flotation

Digestion– aerobic, anaerobic

Mechanical Dewatering– Vacuum filtration, centrifugation, pressure filtr.

Disposal– land application, burial, incineration

Anaerobic Digestion

• Sludge held without aeration for 10-90 daysSludge held without aeration for 10-90 days• Process can be accelerated by heating to 35-40Process can be accelerated by heating to 35-40ooCC• These are called High Rate Digestors (10-20 days)These are called High Rate Digestors (10-20 days)• AdvantagesAdvantages

•low solids productionlow solids production•useable methane gas produceduseable methane gas produced

• DisadvantagesDisadvantages•high capital costshigh capital costs•susceptibility to shocks and overloadssusceptibility to shocks and overloads

Sludge Dewatering

Sludge drying beds– historically the most common– sand bed, 15-30 days, evaporation & seepage

Vacuum Filtration– cylindrical rotating drum covered with fabric– submerged with applied vacuum

Continuous belt filter presses (follows) Plate pressure filters

– vertical plates mounted on a frame

Belt Filter Press (Komline-Sanderson)